In a medium sized bowl combine the graham cracker crumbs, melted butter and sugar. Stir until all the graham cracker crumbs are damped by the melted butter.

Divide evenly between the three pans and press down firmly with your hand or the back of a spoon.

Bake for 8-10 minutes, until golden brown. Let cool while you make the banana cake batter. 

Preheat your oven to 350 degrees F. 

In a medium bowl, sift the flour, baking powder, baking soda and salt. Set aside.

In a stand mixer fitted with the paddle attachment, beat the butter, oil, and sugars on medium-high speed until pale and fluffy, about 5 minutes. Lower the speed to medium and add the eggs, one at a time, mixing until each is fully incorporated before adding the next (about 30 seconds). Add the vanilla.

With the mixer on low, add the dry ingredients alternating with the buttermilk mixture into the creamed mixture, starting and ending with the dry ingredients. Mix until just incorporated and remove bowl from stand mixer. Using a large stainless steel whisk, stir/whisk the mashed banana into the mixture--you can use use some strength and blend well, ensuring all of the ingredients are well mixed. (If you don't have a large stainless whisk, you can fold the banana mixture in using a rubber spatula until incorporated.)

Divide the batter evenly between the three pans that have the graham cracker crust in them, using a kitchen scale for the best accuracy. (For this recipe, you'll have about 1.4 lbs of batter in each pan). 

Carefully smooth the batter with small offset palette knife, and bake until a toothpick or skewer comes out clean, about 28 to 33 minutes. 

Let pans cool on wire rack for 10 minutes, then invert cakes onto racks, gently, peeling away parchment rounds. Let cool completely. 

When the cakes are completely cooled, level the cake layers if necessary

The cake layers can be made up to a week in advance. If you plan to make it in advance, cover each cake layer with two layers of plastic wrap. When you're ready to frost your cake, take the cake layers out of the freezer an hour before you decorate.

Place the marshmallows on a cookie sheet lined with parchment paper and sprayed with nonstick cooking spray. Place on the middle rack of the oven, and broil the marshmallows until golden brown on top, about 30-60 seconds. Keep your eye on them!

Remove the pan from the oven and gently turn the marshmallows over and broil them again until they are golden brown on the other side.

In the bowl of a stand mixer fitted with the paddle attachment, combine the butter and powdered sugar and beat on low until blended. Add the vanilla and mix on medium speed for about three minutes

With the mixer on low speed, add the marshmallow cream and (cooled) toasted marshmallows, and mix for about one minute. 

Combine the butter and Nutella and beat on medium-high speed for 2-3 minutes.

Add the sifted confectioners' sugar, a little at a time until incorporated. Beat for another 2-3 minutes.

Add remaining ingredients and beat for another two minutes, until light and fluffy. Best used right away. 

Spread a small amount of frosting on the cake board. Place the first cake layer, graham cracker side down, on the cake board.

Spread about 1/2 cup of the Nutella frosting on the cake layer and then pipe a rim of frosting around the edge of the cake.

Carefully spread about 3/4 to 1 cup of the marshmallow filling on the frosting.

Place the next cake layer on top of the marshmallow filling and repeat steps 2 and 3.

Place the final cake layer, graham cracker side down, on top of the marshmallow filling. Apply a thin coat of Nutella frosting over the entire cake to lock in the crumbs. Freeze the cake for 10 to 15 minutes to set the crumb coat.